1. Field of the Invention
The present invention relates generally to an optical information recording and reproducing memory system which records, reproduces or erases information data by means of an irradiation of a light beam onto a data storing medium.
More specifically, the present invention relates to an automatic light power compensation system for the above described optical memory system whereby the emitted power of a light beam from a light source is automatically compensated for the fluctuation from a suitable level, and a stable light beam can be maintained.
2. Description of the Prior Art
In an optical information recording and reproducing memory system, information data is recorded, reproduced or erased by means of a light beam which is irradiated onto a data storing medium such as an optical disk.
In general, a data storing medium of this kind, e.g. thermoplastics or magneto-optical magnetic film, shows a non-linear light sensitive characteristic.
On the other hand, a laser diode or other kind of laser oscillator is generally used for the light source in the above described optical memory system. However, these kinds of light sources have their own temperature characteristics of oscillating laser power. Furthermore, there is a degradation of the laser power because of the degradation of the laser source itself.
Under the above mentioned conditions, an automatic light power compensation system is widely used for the optical memory system, whereby a laser power from the light source is automatically compensated for the power fluctuation and stable information recording, reproducing or erasing can be performed.
For example, as disclosed in U.S. Pat. No. 4,692,606 to Sakai and Yoshikawa, a light flux or beam from a laser diode as a light source as detected by a p-i-n photo diode whereby the light power level or light beam intensity of the light source is converted to a power monitor signal. A window type comparator with the upper and lower threshold reference levels is supplied with the power monitor signal and produces a compared output signal. The comparater provides a down-count signal to an UP-DOWN counter when the power monitor signal exceeds the upper threshold reference level, and the comparator provides an up-count signal to the UP-DOWN counter when the power monitor signal is less than the lower threshold reference level. Thereafter, the digital output signal of the UP-DOWN counter is converted to the analog signal by means of a D/A converter. Therefore, the power level of the laser beam from the laser diode is controlled by the analog signal so that the laser power is compensated for the fluctuation from the suitable power level. In other words, a laser power compensation is performed by means of the closed servo control system in which the power monitor signal controls the laser drive current automatically.
Namely, the laser power compensation in the prior art system is performed by the closed servo loop wherein, at first, the light source has to emit the light beam, and then a part of the light beam is detected and the drive signal for the light source is controlled so that the desired stable output light power can be maintained from the light source.
According to the prior art system, only when the light power level remains within a controllable range can a stable control of the light power level depending on the power fluctuation be performed.
However, when the light power level is out of the controllable range, the light power compensation system in the prior system art can not follow the rapid fluctuation in the light power level, and the power compensation system can not work till the power monitor signal enables the closed servo loop.
Furthermore, when a data storing medium is mounted on the memory system, and due to the fact that the compensation system controls the light output only within the controllable range, an uncontrolled light beam may irradiate the data storing medium.
In any case, according to the prior art system, a defective information recording or a burst in the data storing medium may be caused in the optical memory system because of lack of reliability in the power compensation system.